"Deviated" and/or "horizontal" wells, or portions of wells, are frequently drilled using a downhole motor run on coiled tubing. Coiled tubing offers advantages when underbalanced drilling, when drilling through slim boreholes and when drilling through completions. The term "deviated" as used herein refers to any well with a significant deviation from the vertical, such that gravity can create a "cuttings bed" problem of any dimension.
A downhole drilling motor is typically powered by drilling fluid pumped down the drill string. The drilling fluid is pumped through the motor, which thereby powers a drilling element or bit, and out through the bit to cool the bit and to remove drill cuttings by recirculation uphole.
Drilling deviated and especially "horizontal" wells can result in a "cuttings bed" problem. This "cuttings bed" problem is not encountered in this form in vertical wellbores. Cuttings in deviated or horizontal wells, even though carried by the drilling fluid away from the bit tend to settle eventually beneath the drill string in a deviated or horizontal segment. This problem is largely avoided with a surface drive drill string by the constant turning of the drill pipe in the hole. The cuttings removal process is further enhanced with tubulars by the flex of jointed pipe between the joints during rotation.
Cuttings form what is referred to as "cuttings beds" on the lower side of non-rotating drill string in deviated portions of a wellbore. Buildup of "cuttings beds" leads to undesirable friction and possibly to the sticking of the drill string. In coiled tubing drilling, to illustrate the problem, a typical horizontal well may have a diameter of 43/4 inches. A coiled tubing drill string may have a typical outside diameter of 27/8 inches. There is not, therefore, a great expanse of area in which a cuttings bed can freely collect without obstructing the freedom of movement needed for the drill string. Thus, any cuttings bed should periodically be removed when drilling deviated wells with coiled tubing.
When drilling fluid is used to power a downhole drilling motor, the motor itself places limits on the permissible range of fluid flow rates therethrough. Excessive flow rates could damage the motor. The flow rate limits, thus, of a downhole drilling motor are generally considered to place one set of limits on the maximum fluid flow rate permissible down the drill string.
More importantly, with coiled tubing used as the drill string, the surface pressure used for pumping fluid down the tubing if the tubing is being reeled and unreeled places a second, perhaps more significant, set of limits on the maximum fluid flow rate desirable down the drill string.
Tubing suitable for drilling, coiled on a reel of approximately 10 feet in diameter, has been plastically deformed beyond its "yield point". Reeling and unreeling require bending the tubing. Bending shortens the tubings' useful life. Reeling coiled tubing under a significant pressure differential aggreviates the effects of bending on the "fatigue life" of the tubing. The greater the pressure differential the greater the adverse effect on fatigue life. This reduced fatigue life is reflected, in turn, in the cost for a job. The higher the differential pressure under which the tubing will be bent, the significantly shorter the overall useful life of the string. An operation that significantly reduces the lifetime of the tubing will be directly reflected in increased costs. As a consequence, significantly increasing internal coiled tubing pressure at surface, while simultaneously reeling the tubing, as in drilling, may improve fluid flow rate downhole, and may improve cuttings return, but can have a significantly adverse effect upon the cost of a job.
Given the above considerations, the present practice in the industry to cure the "cuttings bed" problems in "horizontal" wells is to perform "wiper trips". For a "wiper trip" the drill string is pulled back along the well, pulling the bit through the horizontal section of the well. Dragging the bit stirs up cuttings from any "bed" and permits the drilling fluid to transport the cuttings up the well. However, dragging the bit can damage its gauge side and dragging the bit while rotating further reams the hole. And although wiper trips can cure a "cuttings bed" problem, they are expensive in the time and equipment they consume. In some wells wiper trips can consume 50% or more of the time of drilling. At a cost of up to $40,000 per day for a coiled tubing drilling rig and associated equipment, this is a significant expense. Alternate solutions are valuable.
The use of muds with special viscosifiers is also practiced in the industry to enhance a "cuttings transport" characteristic of a drilling fluid. However even with specially viscosified drilling fluids, cuttings still settle to form a "cuttings bed" in horizontal wells drilled with a downhole motor. Wiper trips are still required. Thus, although improving the cuttings carrying characteristics of a drilling fluid can delay the settling rate of cuttings, it will not eliminate a cuttings bed from forming in time.
To add a further complicating factor, the use of such special viscosifiers may not always be possible. Horizontal drilling may be performed "under-balanced". Although drilling is typically performed "over-balanced," where a drilling fluid is selected such that the hydrostatic head from the fluid "over-balances" the pressures expected from any downhole formations, "under-balanced" drilling is a growing practice, particularly in horizontal wells because it can be less damaging to sensitive formations. In "under-balanced" drilling the hydrostatic head of the drilling fluid is designed to be exceeded by the pressures expected from the formations downhole. Under-balanced drilling is typically achieved by adding a gas such as nitrogen to a drilling fluid such as water. Drilling under "under-balanced" conditions further limits the ability to maximize a cuttings transport characteristic of a drilling fluid by adding viscosifiers.
A key aspect of the present invention includes establishing, and using apparatus to establish, in a wellbore for a significant period of time a high enough fluid flow rate to create a "critical level" of flow for fluids transporting cuttings through at least a deviated or horizontal portion of the wellbore. Study of cuttings beds problems shows that if a fluid transporting cuttings achieves what is referred to as a "critical" level of flow, a flow that may for instance exhibit a critical level of momentum transfer, especially if this critical level of flow occurs while further cuttings are not being created, then essentially all of a "cuttings bed" can be cleared from a horizontal wellbore in quite a competitive period of time. It has been estimated, for instance, that a removal rate of approximately one foot per second (or sixty feet per minute) can be achieved by "critical flow" without drilling. Ceasing drilling during this period of critical level of flow is compatible both with not creating further cuttings and with not reeling coiled tubing when it is placed under quite high deferential pressure at surface.
Pulling a drill string for a wiper trip typically does not proceed at a rate greater than fifty feet per minute, and usually proceeds slower. Also further time is consumed with wiper trips in returning the string to the drilling position. Hence, removing cuttings using a critical-level-of-flow method offers the promise of saving valuable time. Further, using a critical-level-of-flow method offers the advantage of avoiding wear and tear on the drill string and bit occasioned by pulling in and out with wiper trips, and offers the advantage of not reducing further the lifetime of the coiled tubing by reeling it in and out in a wiper trip, at whatever differential pressure.
Indications are that a "critical level" of flow for drilling fluid in a horizontal well typically occurs at a rate of 3 to 5 feet-per-second. Such a flow rate raises three problems which the instant invention addresses. This critical level of flow is frequently above the maximum flow rate prescribed for fluid flow through a downhole motor. Establishing the critical level of flow may exceed the capacity of the drilling fluid pump. And most importantly, the critical level of flow typically requires surface pressures that would significantly shorten the normal lifetime of coiled tubing if applied while unreeling the tubing.
The instant invention includes, therefore, in preferred embodiments, a downhole circulating valve of adequate size for bypassing at least a portion of the fluid pumped downhole around the drilling motor, preferably into the region of the wellbore proximate the motor. The instant invention also includes holding coiled tubing stationary during periods of increased fluid flow rate. In such cases, all or a majority, of the fluid would be preferably directed to bypass the drill motor. The apparatus also includes in a preferred embodiment being prepared to use at least two pumps located at the surface connected to the coiled tubing such that surface pressure on the drilling fluid can be increased at least two-fold. (The relationship of surface pressure to wellbore fluid flow rate is discussed subsequently.) By so significantly increasing the pressure on the drilling fluid at the surface, and by diverting at least a portion if not all, of the fluid flow downhole from or around the drill motor, and, in addition by avoiding unreeling the coiled tubing during periods of unusually high surface pressure used to achieve the elevated flow rate downhole, circulating fluid can be used effectively to remove cuttings beds.
Studies indicate that if fluids of either the same composition as the drilling fluid or of an alternative composition, are pumped in a deviated or horizontal portion of a wellbore at at least 120% of the fluid flow rate typically used for drilling, such pumping produces wellbore flow rates at a "critical" level. Such flow rates result in a comparatively rapid removal of "cuttings beds" from a horizontal wellbore, especially if drilling is discontinued and no new cuttings are being created. Not only can "cuttings beds" thereby be removed without wiper trips but also the rate of removal of the beds can exceed that of wiper trips, e.g. approximate a linear foot a second. Studies indicate that increasing the flow rate of fluid into the wellbore from 20% to 50% of the normal drilling flow rate will increase the rate of removal of cuttings beds from 2 fold to 4 fold.
Coiled tubing drillers will likely provide and have on hand a primary drilling fluid pump with horsepower and pumping ranges that comfortably exceed anticipated drilling needs. A back up pump should also be on site. The present invention anticipates the possibility of mobilizing both of these pumps in order to achieve the heightened pumping rates on surface necessary to generate a critical flow of drilling fluids in a deviated or horizontal portion of a wellbore. As a rough rule of thumb, increasing flow rate in a horizontal portion of a wellbore by a multiple of X should require an X.sup.2 increase of pump pressure on surface and an X.sup.3 increase of horsepower in the pump. Thus, raising a downhole flow rate from R to 3/2R may require raising the pressure of the drilling fluid on surface from P to 9/4P, or to 2.25P, which may require a pump horsepower of 27/8 HP, or about 3.4 HP, where HP is the normal horsepower used for drilling (e.g. used for producing pressure P and flow rate R). The availability and mobilization of larger than normal pumps and/or multiple pumps, thus, comprises an aspect of the apparatus disclosed herein.
A coiled tubing drill string when drilling a horizontal portion of a well will likely predominatly lie, due to gravity, on the lower side of the horizontal wellbore. During drilling the weight of the coiled tubing will be "held" at surface and "managed" to maximize drilling performance, or rate of penetration. Only partial weight, typically, is "set down" on the bit while drilling. While not drilling, the weight-on-bit can be managed to enhance cuttings bed removal. Cuttings bed removal in a horizontal portion of a wellbore may be enhanced if the string is encouraged to helix in the wellbore rather than to lie predominantly on the lower side of the wellbore. Helixing of coiled tubing in the wellbore may be encouraged by managing the weight-on-bit, and in particular by likely setting down more weight. One aspect of the present invention involves managing the weight-on-bit to enhance cuttings bed removal.